"When we looked out at that, there is no sitting on the sidelines, there is no hoping that somebody else can get this done," O'Rourke said.

I am running to serve you as the next president. The challenges we face are the greatest in living memory. No one person can meet them on their own. Only this country can do that, and only if we build a movement that includes all of us. Say you're in: https://t.co/EKLdkVET2upic.twitter.com/lainXyvG2n

The campaign will test whether the former congressman can replicate his insurgent run in Texas — he nearly upset Cruz — on the national stage, while facing off against a cast of Democrats that includes sitting senators, current and former governors and possibly a former vice president.

At his first campaign stop, O'Rourke introduced himself to a crowd at a coffee shop in Keokuk, Iowa, saying he already missed his kids and that he hoped to speak with people of different political persuasions across Iowa and the country.

"There's no sense in campaigning if you already know every single answer, if you're not willing to listen to those whom you wish to serve," O'Rourke said.

The announcement that O'Rourke is running comes after months of speculation over what the Democrat was planning following his loss to Cruz by less than three percentage points in the 2018 Senate race. The 46-year-old came closer to unseating an incumbent Republican senator in the state than anyone since Democrat Bob Krueger narrowly lost to Republican John Tower in 1978, according to The Texas Tribune.

The race for the Democratic presidential nomination in 2020 is underway and wide open. Those who have filed paperwork or announced presidential bids include senators, House members and former lawmakers.

O'Rourke set a self-imposed deadline about his future plans in a Feb. 5 interview with Oprah Winfrey, saying he would make a decision before the end of February, and that he had "been thinking about running for president."

Six days later, with President Donald Trump in O'Rourke's hometown of El Paso, O'Rourke spoke at a counter-rally within shouting distance of the coliseum where the president spoke.

"My short answer is no. I don't think it's disrespectful," O'Rourke said in the clip.

After he paid respect to veterans and thanked civil rights leaders who brought about change, O'Rourke said, "Non-violently, peacefully, while the eyes of this country are watching this game, they take a knee to bring our attention and our focus to this problem to ensure that we fix it. This is why they are doing it and I can think of nothing more American then to peacefully stand up or take a knee for your rights anywhere, any place, anytime."

The clip drew attention from celebrities ranging from LeBron James to Ellen DeGeneres, who invited O'Rourke to appear on her show later in the campaign.

During the race, O'Rourke's nickname -- "Beto" -- came under fire from the Cruz campaign. The former congressman's birth name is Robert Francis O'Rourke.

"My parents have called me Beto from day one, and it's just -- it's kind of a nickname for Robert in El Paso. It just stuck," O'Rourke told CNN in 2018.

Aside from going after him for his nickname, Texas Republicans tried to use the fact that O'Rourke was in a punk rock band called Foss when he was in his early 20s against him, in addition to more serious allegations.

The official Twitter account of the Texas Republican party resurfaced a mugshot of O'Rourke from the 1990s. He was arrested twice for misdemeanors in his 20s, but was not convicted.

There's always the chance that Robert “Beto” O’Rourke won’t debate Senator Cruz because he got into a hazy situation... pic.twitter.com/4nmd42AEkl

In his three terms in the House, O'Rourke's voting record was more conservative than most of his Democratic colleagues, according to Voteview.com, a site run by UCLA's Department of Political Science. In the 113th Congress (2013-15), O'Rourke was more conservative than 76 percent of House Democrats; in the 114th Congress (2015-17), he was more conservative than 79 percent of Democrats; and in the 115th Congress (2017-19), he was more conservative than 77 percent of House Democrats.

In 2012, O'Rourke unseated eight-term incumbent Silvestre Reyes in the Democratic primary and won the general election to take Texas' 16th congressional district. Prior to serving in Congress, O'Rourke was on the El Paso City Council from 2005-11.